If someone interested - I've just found solution for subject question:

1. Add to the Infopath form field to store library title, name it "Title"
for example
2. Define managed OnLoad event handler
3. Make web reference for Sharepoint's Lists web service:
http://<sharepoint>/sites/<site>/_vti_bin/Lists.asmx?wsdl
4. OnLoad handler will look like:
[InfoPathEventHandler(EventType=InfoPathEventType.OnLoad)]
public void OnLoad(DocReturnEvent e)
{
 try
 {
   Lists ws = new Lists();
   ws.Credentials = CredentialCache.DefaultCredentials;
   XmlNode sps = ws.GetListCollection();

   XmlNamespaceManager mngr = new XmlNamespaceManager(
sps.OwnerDocument.NameTable );
   mngr.AddNamespace( "def",
"http://schemas.microsoft.com/sharepoint/soap/" );

   string solutionUri =
HttpUtility.UrlDecode(thisXDocument.Solution.URI.ToString());
   string uri = new Uri(HttpUtility.UrlDecode(solutionUri),
true).PathAndQuery;

   XmlNode lib = sps.SelectSingleNode( "//def:List[@DocTemplateUrl='" + uri
+ "']/@Title", mngr );
   IXMLDOMNode num = thisXDocument.DOM.selectSingleNode( "//my:Title" );
   if ( lib != null ) num.nodeTypedValue = lib.InnerText;
 }
 catch
 {
   thisXDocument.UI.Alert( "Can't get library title");
 }
}